Currently, standardization of fourth-generation mobile communication systems is performed in third generation partnership project (3GPP). “Long term evolution (LTE)”, that is one of data communication specifications designed by 3GPP, is a long term advanced system aiming for the fourth generation (4G) IMT-Advanced, and is called “3.9G (super 3G).”
In LTE, two duplex systems, i.e., frequency division duplex (FDD) and time division duplex (TDD) can be selected. In the FDD, an uplink-dedicated band and a downlink-dedicated band are used. In the uplink and the downlink, a format of a radio frame composed of ten consecutive sub-frames is used. Here, the uplink refers to communication from a terminal station (UE terminal: user equipment) to a base station (eNodeB: evolved Node B) of LTE, and the downlink refers to communication from an eNodeB to a UE terminal. In the TDD, a format of a radio frame composed of 10 consecutive sub-frames is also used. However, in the TDD, communication is performed using the same band in the uplink and in the downlink. Each sub-frame of the radio frame is composed of a control signal phy downlink control channel (PDCCH) from an eNodeB and a phy downlink shared channel (PDSCH) that is used as user data.
Coordinated multi point transmission and reception (CoMP) is considered as a key technology for improving the throughput of UE terminals positioned at cell edges in the standardization of Release 11 related to LTE of 3GPP. The CoMP is a technology for increasing desired signal power and for reducing interference from another cell by simultaneous data transmission and reception of plural eNodeBs with respect to one UE terminal. In order to efficiently derive macro diversity gain by CoMP, precoding, a reference signal, a measurement and feedback method, and the like should be examined. In LTE, a method of hierarchizing various sizes of cells such as macro, micro, pico and femto cells, called HetNet, is examined in Release 8, and the CoMP also includes communication to femto cells. The CoMP includes uplink CoMP and downlink CoMP, and the uplink CoMP is an important technology as the downlink CoMP. However, unless otherwise noted, description will be given regarding the downlink CoMP.
Examples of the method related to cells that realize the CoMP include a method in which plural eNodeBs respectively perform autonomous distributed control and a method in which one macro eNodeB performs integrated control on plural pico eNodeBs. In the latter integrated control method, plural base stations such as remote radio heads (RRHs) are arranged as a measure for non-sensing in cell edges and the like, and the connection with a macro eNodeB that performs integrated control of the base stations is achieved using an optical fiber with a baseband signal (to be described later). The macro eNodeB performs baseband signal processing and control of the respective RRHs to perform collective radio resource control between the cells.
Regarding the Determination of a CoMP Set
A combination of eNodeBs that perform CoMP with respect to one UE terminal, that is, that constitute a cooperation group will be called a set of CoMP transmission point(s) or a CoMP set for short.
In order to determine a CoMP set, it is necessary to judge which eNodeB is the most effective one to use for a UE terminal. This will be called CoMP set determination or point selection.
As one method for point selection, a method including receiving a reference signal from each base station by a UE terminal to measure reference signal received power (RSRP) for each base station, and selecting eNodeBs positioned at an upper level thereof as a CoMP set is exemplified.
For example, a cooperation group setting method in which a base station transmits, to a user terminal, cooperation group-setting signaling to which a cell ID of a cell in a cooperation group selected for the user terminal is attached, and the user terminal performs cooperation group setting by the cell ID of the cell in the cooperation group selected for the user terminal attached to the cooperation group-setting signaling is proposed (for example, see PTL 1).
In addition, a radio communication method in which a base station allocates a part of an entire frequency band to bands that are respectively used in single base station transmission (first transmission method) and in multiple base station transmission (second transmission method) to determine to use which transmission method based on feedback information indicating receiving quality received with respect to either of the transmission methods, that is, to reduce a feedback information quantity by limiting the feedback information to information indicating receiving quality of a part of the entire frequency band is proposed (for example, see PTL 2).
In addition, a radio communication system in which each of base station devices receives, from a target terminal device, quality information indicating communication quality between each base station device and a base station device communicable with the target terminal device, and acquires schedule information indicating a schedule of communication with a terminal device existing in the cell of another base station device to select, based on the quality information and the schedule information, several base station devices that are allowed to function as a base station with respect to the target terminal device is proposed (for example, see PTL 3).
Needless to say, determination of an eNodeB required for a UE terminal when performing point selection should be considered, but it is also important not to use an excessive number of eNodeBs in CoMP. The reason for this is that when an excessive number of eNodeBs are included in a CoMP set, the throughput of the entire system is reduced and the radiation of unnecessary radio waves to the neighborhood is increased. For example, in a case in which sufficient transmission quality is obtained for a UE terminal when, as a CoMP set, an eNodeB#1 and an eNodeB#2 perform downlink transmission, when an eNodeB#3 is further added to the CoMP set, the eNodeB#3 radiates unnecessary radio waves to the neighborhood, and thus becomes an interference source. Accordingly, it is important to constitute the CoMP set with a minimum necessary number of eNodeBs satisfying necessary quality.
Request Related to Point Selection Frequency
It is necessary to perform the point selection every certain period of time (that is, it is necessary to update the CoMP set every certain period of time). That is, whether the CoMP set of the UE terminal is to be semi-statically or dynamically updated becomes a problem. When considering the movement of the UE terminal, it is desired to perform dynamic point selection to dynamically update the CoMP set.
Basic Points to be Considered in Dynamic Point Selection
When performing dynamic point selection, it is necessary to consider a reduction in throughput due to the occupancy of a communication sequence by reference signals in the system, in other words, a downlink overhead due to reference signals.
In addition, it is necessary to consider an increase in power consumption of the UE terminal due to the frequent measurement of the reference signals by the UE terminal.
It is also necessary to consider an increase in uplink overhead due to feedback of the reference signal measurement results from the UE terminal to the eNodeB.